The present invention relates to the control of servo loops and, in the preferred embodiment, the simultaneous rapid open and closed loop control measurements obtained using a binary pseudo-random sequence.
The use of a binary pseudo-random sequence to test an electrical or mechanical system for the purpose of computing its power spectrum by discrete fast Fourier transform (DFT) techniques is well known, for example, as described in U.S. Pat. No. 4,093,988. In accordance with U.S. Pat. No. 4,093,988, the properties of binary pseudo-random noise and the discrete fast Fourier transforms are utilized to, in effect, simultaneously inject a closely spaced set of equal amplitude sign waves over a prescribed frequency range into the system under test and to separate the response of the output. For its discussion of binary pseudo-random sequences and filters therefore, and discrete fast Fourier transform techniques employed therein, the U.S. Pat. No. 4,093,988 patent is incorporated herein by reference. Still, the '988 patent does not describe the use of binary pseudo-random sequences within feedback devices, for simultaneous rapid open and closed loop bode plot measurement in servo loops, or in general, any loop calibration application.
Within the context of servo loop control, existing techniques to generate a bode plot measurement inject a swept sinusoid, random white noise, or a chirp into the system under test in order to obtain the bode plot measurements. A method and apparatus for tuning motion control system parameters using random noise excitation, for example, is described in Yutkowitz, U.S. Pat. No. 6,259,221. In the '221 patent, the system comprises a random noise signal generator in communication with the servo controller, and a frequency response analyzer in communication with the servo controller. The random noise signal generator is configured to provide a random noise signal to the servo controller during controlled motion of the actuator. The frequency response analyzer is configured to generate frequency responses for the random noise signal. The '221 patent does not specify the type of random noise used beyond the generalities, “white noise,” “pink noise,” or “band limited white noise signals.”
Thus, the previous binary pseudo-random sequence test apparatus, such as U.S. Pat. No. 4,093,988, uses only the output measurement of a system under test and cannot generate an open loop or closed loop bode plot, which would require both input and output of the system under test. Other devices, such as U.S. Pat. No. 6,259,221, describe the calculation of open loop and closed loop responses, but fail to recognize the value of binary pseudo-random sequences used to generate a bode plot.
The present invention injects a binary pseudo-random sequence, using both the input and the output of the system under test to generate bode plots. The invention has the advantage of generating the open and closed loop bode plots while the system under test is operating closed loop. This allows real-time bode plots to be computed for in-situ applications, such as on-orbit applications, while simultaneously maintaining control of the system under test. The advantages of the present invention are significant. The invention produces measurement results approximately 1,000 times faster than a swept sinusoid approach and has superior dynamic range as compared to using random white noise as a test input source.